Devices for automatically delivering small predetermined quantities of liquid soap or the like onto the hands of a user without requiring manual operation thereof or any physical contact therewith are known in the prior art. Such devices typically include a container for supporting the soap, a mechanical actuator for dispensing the soap from the container, and electrical control means for controlling the operation of the mechanical actuator. One such device is shown in U.S. Pat. No. 3,327,901 to Yerkovich. The electrical control means in the Yerkovich patent includes an a.c.-powered incandescent lamp which cooperates with a photoconductor cell. As long as a light beam between these elements remains uninterrupted, the mechanical actuator is deenergized. Once the beam is interrupted by the user's hands, however, the circuit activates the mechanical actuator to dispense the soap.
More recently, attempts have been made to provide improvements to the Yerkovich-type dispenser by incorporating battery-operated electronic control circuitry into the device. One such device is described in U.S. Pat. No. 4,722,372 to Hoffman et al. In particular, the Hoffman et al. patent discloses an automatic soap dispenser which includes a proximity sensor to detect the approach of a user. The proximity sensor is designed to activate a normally-inactive photocell sensing system which in turn establishes a pulsed trigger signal between a light emitting diode and a phototransistor. As in other art "thru-scan" techniques, the interruption of the trigger signal by the user's hands controls the actuation of a mechanical actuator for dispensing the soap. The Hoffman et al. patent also describes a disposable soap container which includes a built-in battery pack for energizing the dispenser.
While the Hoffman et al dispenser provides advantages over earlier designs by requiring only selective activation of the photocell sensing system, this device still requires a continuous supply of power to the proximity sensor to provide proper operation of the device. The proximity sensor circuitry causes a continuous drain on the battery supply, and this power drain is exacerbated when the photocell sensing system is activated to produce the pulsed trigger signal. The battery power drain reduces the effective life of the built-in battery pack disposable container, thus increasing the cost of maintaining the device.
It is also known in the prior art to provide a proximity detector which uses an a.c. signal-modulated trigger waveform to minimize the effects of ambient light on the photocell sensing system. These systems typically require the use of a sharply tuned filter to separate the modulating signal from the background noise. Such modulation schemes are therefore incompatible with a battery-powered dispenser because of the continuous power requirements of the emitter and detector circuitry.
Other prior art fluid dispensing devices include sophisticated electronic circuitry for receiving a composite signal including both pulsed infrared light and ambient light and then biasing out the ambient light and passing only the infrared pulses to a processing circuit. This approach is disclosed in European Patent Application No. 0,078,181 assigned to Calgon Corporation. European Patent Application No. 0,127,497, also assigned to Calgon Corporation, discloses a similar device which includes additional circuitry that shuts down the dispenser electronics when the room is dark. While these devices have proven satisfactory, the electronic circuitry therein is complex and therefore expensive.
It would therefore be desirable to provide an improved electronic control means for use in an automatic material dispenser of the type generally described above which exhibits low average power drain yet provides significant immunity to background noise.